A vacuum servo-unit for reducing the operating input force to be applied when the operation of the vehicle braking system is initiated has been generally used for some time. This vacuum servo unit typically includes a constant pressure chamber to which the negative pressure is constantly supplied, and a variable pressure chamber. In one state, the variable pressure chamber is communicated with the atmosphere and is cut off from the constant pressure chamber, and in another state the variable pressure chamber is communicated with the constant pressure chamber and is cut off from the atmosphere. A vacuum valve member intermittently establishes communication between the constant pressure chamber and the variable pressure chamber, and an air valve member intermittently establishes communication between the atmosphere and the variable pressure chamber. The vacuum valve member and the air valve member are respectively opened and shut by the operation of the brake operation member, and a pressure differential according to the operation force supplied to the brake operation member is established between the constant pressure chamber and the variable pressure chamber. Thus, the vacuum servo unit generates an amplified brake force according to the operation force supplied to the brake operation member.
In recent years, a vacuum servo unit employing a solenoid valve operated by electricity has been known. This type of vacuum servo unit is disclosed in Japanese Laid-Open Publication No. 5-24533. In this vacuum servo unit, when the driver doesn't actuate the brake operation member, the solenoid valve is operated so that the variable pressure chamber is communicated with the atmosphere by the operation of the solenoid valve. As a result, a big pressure differential is generated between the variable pressure chamber and the constant pressure chamber and so a big braking force is generated.
However, the foregoing vacuum servo unit has a passage at a center portion thereof through which flows atmospheric air. This decreases the area receiving the pressure differential of the movable member which generates the servo force. This results in reduced servo efficiency.
Additionally, the inhalation port of the passage through which flows the atmospheric air is forced to be provided on the limited portion located at the front of the vacuum servo unit. This restricts the placement of the vacuum servo unit when the vacuum servo unit is installed in the vehicle.
When an urgent state is detected from, for example, the operation speed of the brake operation member so that the vacuum servo unit generates a braking force that exceeds the normal operation braking force, atmosphere is introduced into the variable pressure chamber regardless of the operation of the brake operation member. Therefore, the vacuum servo unit must be provided with a detecting device for detecting a return operation of the brake operation member in case the brake force is released when the driver applies a return operation of the brake operation member.
In an urgent condition, a considerable amount of atmospheric air must be introduced into the variable pressure chamber in order that the maximum servo force is generated. Thus, it takes a relatively long time to increase the brake force.